1. Field of the Invention
This invention relates to data storage apparatus, and in particular, to a track following servosystem useful in disk files.
2. Description of the Prior Art
Although the system described in U.S. Pat. No. 4,072,990 represents a marked improvement over previous systems, it has a disadvantage caused by the cyclic nature and hence the non-linearity of the position information signals, or position error signals (PES), used to control the servo. The problem exists as a result of disturbances which affect the mechanical stability of the apparatus and can lead to the servo head being considerably displaced from its true on-track position over the associated servo track, despite the data head being accurately positioned over the data track being followed.
Under these circumstances, it is possible for the servo head to be so far off track that the position information or position error signals being supplied by the servo head are from the non-linear part of the signal. That is, they do not exhibit a linear relationship with respect to displacement from the servo on-track position. Accordingly, since the high frequency components of this signal are required to form the hybrid servo signal used during track following operations, the gain of the servo circuits at high frequencies varies with changing displacement from the servo on-track position. Since these servo head displacements are caused largely by external disturbances such as temperature variations or shocks and vibrations, as well as internal influencing factors such as eccentricity and tilt of disks on the disk spindle, they are largely uncontrollable. The continuous and unpredictable changing in gain of the servo circuits can lead to loop instability and/or increased head settle time, both of which are undesirable features.